Israel's Quadrant: Weeping, Laughing, and the Measures of the Stars

摘要

In the Jewish culture of the Middle Ages, the astral bodies were constantly given a special attention. They were deeply admired in religious poetry, and studied and observed in scientific and technical works. In an elegy of an anonymous poet of the 11~(th) century, which is usually read in the celebrations of the 9~(th) day of Ab, constellations and stars share the grief for the destruction of the temple of Jerusalem. Like the Israel Tribes, all constellations are said to weep; Pleiades and Orion are said to darken their face, the Scales turn asymmetrical, the pan of death overrides the pan of life; Scorpio is scared and trembles, Sagittarius turns his face back. Jacob ben Machir ibn Tibbon, born in Marseille in 1236, was physician, mathematician, and astronomer. He translated Euclides' Elements, the Treatise of the Armillar Sphere by Qusta ibn Luqa, the Treatise on the Astrolabe by abu al-Kasim Ahmad ibn al Saffar, and works of Ptolemy, Averroes, and Aristoteles. His most important work is the short treatise Rova' Israel (“Israel's Quadrant”) which he later translated in Latin with the help of Armengaud from Blaise. In the first chapter, he discusses the Sun risings, the days of the month, the place of the Sun in the sky, and the lunar motion. In the second chapter, he discusses the relations between sunrise and sunset, between moonrise and moonset, and between solar year and lunar month. In the following chapters, he illustrates a technical device which he had developed and which he designates “Israel's Quadrant”. This instrument could be used both for the sky and for the Earth, since it allowed to survey and measure the elevation of the stars, the elevation of terrestrial reliefs, and terrestrial horizontal distances. The quadrant is flat, and its area is that of a quarter of a circumference. This instrument was very versatile, it also allowed to calculate hours and time, and this favoured its exceptional diffusion. Thus, its name, “quadrant”, was transferred to the visible surface of the clock, notwithstanding the fact that its shape is that of a complete circumference.